Changing the voting system will benefit Britain's "progressive majority", Ed Miliband will say as he campaigns alongside senior Liberal Democrats for a "yes" vote in the May 5 referendum.
The Labour leader will share the stage with other supporters of a switch to the Alternative Vote (AV) system, including ex-Lib Dem leader Charles Kennedy and Green Party MP Caroline Lucas.
There will be no place though for Deputy Prime Minister Nick Clegg, with whom Mr Miliband has refused to appear publicly, calling him a liability to the campaign and advising him to "lie low".
A planned event which would have brought the Labour leader and Mr Kennedy together was cancelled earlier this month amid reports of behind-the-scenes disagreements over the line-up.
Mr Miliband will use the event to say that the first-past-the-post system had allowed the Conservatives too often to take power because the political left was "divided".
In an appeal to disillusioned Lib Dem activists, he is expected to say: "The tragedy for progressive politics in Britain has been that division on the centre and left has handed a united right victory after victory.
"For most of the last 80 years, there has been one Conservative Party - and others competing with Labour for progressive votes.
"The results, over the years, speak for themselves. No wonder the Tories back the current system.
"They know Britain is not a fundamentally Conservative country. But with first past the post, they too often govern when progressive forces are divided.
"This Tory-led Government and its current alliance of power with the Liberal Democrats does not change my belief that there is a progressive majority in this country."